This invention relates to a system and method of controlling the speed of an AC induction motor and, in particular, to a system using sliding mode control for the control of AC induction motor speed.
Due to present efforts directed to reducing the generation of atmospheric pollutants, the electric vehicle has received increasing interest as an alternative to the vehicle using a conventional internal combustion engine. Today, electric vehicles are beginning to enter the marketplace. These vehicles utilize centrally-located charging stations for recharging the DC power supplies used in the vehicle.
The advantages of an AC motor over the larger DC motor with its relatively complex structural requirements have favored the use of the AC motor as the driving element in the electric motor vehicle. For example, the induction motor needs no direct electrical connection to the rotor, thus eliminating the need for rotating electrical contacts. However, the speed control of the AC motor is recognized as a problem in that a motor controller is required to sense, compute and correct currents in a dynamic system having hysteresis, inertia and other time limiting responses. As a result, the controller of the AC induction motor is continually trying to provide the appropriate current for each phase of a multiphase AC motor under very demanding operating conditions.
Among the different approaches taken to provide effective motor current control is the use of analog comparators to compare actual motor currents with reference currents that are generated by the motor controller. The resultant difference signals are used to control the power supply circuits which generate the phase currents delivered to the phase terminals of the motor. The pulse width of the power supplied to the motor for each phase is a function of the difference signal provided by the analog comparator for each phase. This type of controller uses pulse-width modulated inverter circuits, one for each phase, to supply the power to the motor terminals.
Another approach to the provision of a controller for a multiphase AC motor utilizes digital circuits to generate a digital difference signal for each phase based on a comparison of the actual phase current and a desired phase current computed by a programmable logic circuit. The logic circuit also establishes a band in which the digital difference signals are to be maintained. This type of motor controller requires the sensing of actual phase currents to permit a comparison to be made and to then generate the signals to effectively control the inverter circuits which provide power to the motor phase terminals. This type of controller utilizes both analog and digital circuitry to sense when negative or positive signals are to be supplied to the individual phase windings of the motor.
Accordingly, the present invention is directed to a motor controller for use in connection with an AC motor in which the need to sense the phase currents or the rotor flux in the motor is eliminated. Further, the present invention provides a motor controller for an AC motor in which the only quantities that are monitored are the desired speed and the actual motor speed. The subject controller provides accurate and rapid motor control without relying on the use of analog monitoring components for sensing current or flux in the motor. In addition, the present controller operates to control the motor speed essentially independently of loading of the motor.